A number of techniques have been disclosed for preparing refractory shells for casting of metals. In a common method, a wax pattern is dipped into a refractory slurry consisting of a refractory flour such as zircon or fused silica in colloidal silica sol or in a hybrid binder composition. The coating produced in stuccoed with refractory particles or aggregate, for example, by dipping the coated pattern into a fluidized bed of the particles or by sprinkling the pattern with the particles. The dipping and stuccoing steps are repeated with drying after each stuccoing step. The major disadvantage of this technique is the long drying times required which can sometimes range up to about 72 hours per coat depending, for example, upon the thickness of the coating employed, the number of prior coats, the materials employed in the slurries, and the method of drying such as in forced air drying tunnels. Such methods require extensive drying equipment, labor, time and space, if a reduction in drying time is desired. Also, during the time that the coatings are soft and pliable, they are difficult to handle. Moreover, the drying of such coatings is subject to various temperature and humidity changes which can affect the precision of the refractory shell produced. In particular, because the moisture content can vary in the coating during the drying, it is difficult to maintain the temperature and humidity conditions necessary to avoid expansion or contraction of the pattern, which can result in the loss of the desired precision of the shell. Further, with coatings of colloidal silica sol, a gel is formed on drying, which gel shrinks and such shrinkage can cause undesirable cracking of the refractory, e.g., of the refractory shell, which can in turn cause metal leakage when molten metal is poured therein. Still further, if the underlying coats of colloidal silica sol, ethyl silicate or hybrid binder composition are not adequately dried prior to the application of the next coat thereof, the underlying coats can be rewetted or softened, which can cause the applied coatings to lift off from the surface of the wax mold which in turn causes undesirable bulges or in the extreme, holes in the resulting shell.
Others have attempted to provide chemical methods for setting of binder coatings. Various chemical setting techniques are disclosed in the art. For example, the "Background of the Invention" section of Moore U.S. Pat. No. 3,748,157 discribes a number of such techniques. For example, the Moore patent mentions the chemical setting of sodium silicate-bonded shells containing ammonia by use of carbon dioxide and the use of ammonia to set hydrolyzed ethyl silicate or acidified aqueous colloidal silica-bonded shells. In addition, Moore mentions the use of volatile organic base to set shells bonded with hydrolyzed ethyl silicates. Other approaches mentioned in the Moore patent include the use of an acidified aqueous collidal silica to gel a basic colloidal silica and vice versa and the use of aqueous colloidal silica containing ammonia to set an ethyl silicate dip coat. The Moore patent itself is directed to a process for forming a refractory laminate on the surface of support structure in which the structure is dipped in a bath comprising a colloidal silica sol and/or a solution of alkaline ionic silicate to form a coating on the surface. The coated surface is contacted with a basic aluminum salt setting agent to firmly gel the colloidal silica or silicate.
Another method for chemically gelling of colloidal silica-based binder vehicles is disclosed in Yates U.S. Pat. No. 3,920,578. The Yates patent first points out in the "Background Of The Invention" section the relatively weak bond between layers of colloidal silica sols gelled by adjusting the pH of the sol and by simultaneously adding a neutral salt such as magnesium chloride to enhance destabilization. The Yates patent then discloses a composition and method which is said to improve or reinforce the relatively weak junction points. In particular, the Yates patent discloses a method in which a small amount of water-soluble alkaline ionic silicate is admixed with a colloidal amorphous silica aquasol and the resulting composition is adjusted to a pH of from 5 to 9.5. However, by employing alkaline ionic silicate to set the colloidal silica sol, a relatively high amount of alkaline material is introduced into the investment casting shell, which can reduce the refractoriness of the shell and therefore limit its utility for casting of metals which require high melting temperatures.
Other patents employ processes in which, for example, a pattern is alternately dipped into two refractory compositions, the second of which causes gellation of the coating resulting from such dips. These processes thus proceed via a relatively quick chemical gelling step without an intermediate drying step between the alternate dips of the two refractory compositions. Because gels are formed, it is believed the shells resulting from these process will be subject to a number of disadvantages, for example, contamination of the second refractory composition by the outermost coat of the first refractory composition, slow and perhaps reversible drying of the gel, and shrinkage and consequent cracking of the shells. Examples of such patents include Szabo U.S. Pat. No. 4,019,559; Shaul U.S. Pat. No. 2,806,270; Emblem et al. U.S. Pat. No. 3,270,382; Emblem et al. U.S. Pat. No. 3,292,220; Moore U.S. Pat. No. 3,748,156; Beyer et al. U.S. Pat. No. 3,751,276; Moore U.S. Pat. No. 3,752,679; Moore U.S. Pat. No. 3,752,680; Moore U.S. Pat. No. 3,752,681; Moore U.S. Pat. No. 3,752,689; Moore U.S. Pat. No. 3,754,945; Moore U.S. Pat. No. 3,754,946; Moore U.S. Pat. No. 3,767,458; Beyer et al. U.S. Pat. No. 3,859,153; Moore et al., U.S. Pat. No. 3,860,476; Beyer et al. U.S. Pat. No. 3,878,034; Moore U.S. Pat. No. 3,894,572; Moore U.S. Pat. No. 3,894,572; Taylor U.S. Pat. No. 3,898,313; Emblem et al. U.S. Pat. No. 4,068,701; British Patent Specification No. 731,445; and British Patent Specification No. 1,031,778. Taylor U.S. Pat. No. 3,898,313 is the only one believed to disclose the use of an acidic hybrid binder composition.
It would be highly desirable to provide a refractory coating composition which can be set irreversibly and fairly quickly to a hard coating suitable for an investment casting shell, which will not rewet or soften upon application of a subsequent coat, which minimize shrinkage and its consequent cracks, and which can provide an investment casting shell which can be employed in connection with casting of high temperature melting metals or alloys.